The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.
The class of proteins known as chaperones has been defined by Hartl (1996, Nature, 381, 571-580) as a protein that binds to and stabilises an otherwise unstable conformer of another protein and, by controlled binding and release, facilitates its correct fate in vivo, be it folding, oligomeric assembly, transport to a particular subcellular compartment, or disposal by degradation.
BiP (also known as GRP78, Ig heavy chain binding protein and Kar2p in yeast) is an abundant ˜70 kDa chaperone of the hsp 70 family, resident in the endoplasmic reticulum (ER), which amongst other functions, serves to assist in transport in the secretory system and fold proteins.
Protein disulphide isomerase (PDI) is a chaperone protein, resident in the ER that is involved in the catalysis of disulphide bond formation during the post-translational processing of proteins.
Studies of the secretion of both native and foreign proteins have shown that transit from the ER to the Golgi is the rate-limiting step. Evidence points to a transient association of the BiP with normal proteins and a more stable interaction with mutant or misfolded forms of a protein. As a result, BiP may play a dual role in solubilising folding precursors and preventing the transport of unfolded and unassembled proteins. Robinson and Wittrup, 1995, Biotechnol. Prog. 11, 171-177, have examined the effect of foreign protein secretion on BiP (Kar2p) and PDI protein levels in Saccharomyces cerevisiae and found that prolonged constitutive expression of foreign secreted proteins reduces soluble BiP and PDI to levels undetectable by Western analysis. The lowering of ER chaperone and foldase levels as a consequence of heterologous protein secretion has important implications for attempts to improve yeast expression/secretion systems.
Expression of chaperones is regulated by a number of mechanisms, including the unfolded protein response (UPR).
Using recombinant techniques, multiple PDI gene copies have been shown to increase PDI protein levels in a host cell (Farquhar et al, 1991, Gene, 108, 81-89).
Co-expression of the gene encoding PDI and a gene encoding a heterologous disulphide-bonded protein was first suggested in WO 93/25676, published on 23 Dec. 1993, as a means of increasing the production of the heterologous protein. WO 93/25676 reports that the recombinant expression of antistasin and tick anticoagulant protein can be increased by co-expression with PDI.
This strategy has been exploited to increase the recombinant expression of other types of protein.
Robinson et al, 1994, Bio/Technology, 12, 381-384 reported that a recombinant additional PDI gene copy in Saccharomyces cerevisiae could be used to increase the recombinant expression of human platelet derived growth factor (PDGF) B homodimer by ten-fold and Schizosacharomyces pombe acid phosphatase by four-fold.
Hayano et al, 1995, FEBS Letters, 377, 505-511 described the co-expression of human lysozyme and PDI in yeast. Increases of around 30-60% in functional lysozyme production and secretion were observed.
Shusta et al, 1998, Nature Biotechnology, 16, 773-777 reported that the recombinant expression of single-chain antibody fragments (scFv) in Saccharomyces cerevisiae could be increased by between 2-8 fold by over-expressing PDI in the host cell.
Bao & Fukuhara, 2001, Gene, 272, 103-110 reported that the expression and secretion of recombinant human serum albumin (rHSA) in the yeast Kluyveromyces lactis could be increased by 15-fold or more by co-expression with an additional recombinant copy of the yeast PDI gene (KlPDI1).
In order to produce co-transformed yeast comprising both a PDI gene and a gene for a heterologous protein, WO 93/25676 taught that the two genes could be chromosomally integrated; one could be chromosomally integrated and one present on a plasmid; each gene could be introduced on a different plasmid; or both genes could be introduced on the same plasmid. WO 93/25676 exemplified expression of antistasin from the plasmid pKH4α2 in yeast strains having a chromosomally integrated additional copy of a PDI gene (Examples 16 and 17); expression of antistasin from the vector K991 with an additional PDI gene copy being present on a multicopy yeast shuttle vector named YEp24 (Botstein et al, 1979, Gene, 8, 17-24) (Example 20); and expression of both the antistasin and the PDI genes from the yeast shuttle vector pC1/1 (Rosenberg et al, 1984, Nature, 312, 77-80) under control of the GAL10 and GAL1 promoters, respectively. Indeed, Robinson and Wittrup, 1995, op. cit., also used the GAL1-GAL10 intergenic region to express erythropoietin and concluded that production yeast strains for the secretion of heterologous proteins should be constructed using tightly repressible, inducible promoters, otherwise the negative effects of sustained secretion (i.e. lowered detectable BiP and PDI) would be dominant after the many generations of cell growth required to fill a large-scale fermenter.
Subsequent work in the field has identified chromosomal integration of transgenes as the key to maximising recombinant protein production.
Robinson et al, 1994, op. cit., obtained the observed increases in expression of PDGF and S. pombe acid phosphatase using an additional chromosomally integrated PDI gene copy. Robinson et al reported that attempts to use the multi-copy 2 μm expression vector to increase PDI protein levels had had a detrimental effect on heterologous protein secretion.
Hayano et al, 1995, op. cit. described the introduction of genes for human lysozyme and PDI into a yeast host each on a separate linearised integration vector, thereby to bring about chromosomal integration.
Shusta et al, 1998, op. cit., reported that in yeast systems, the choice between integration of a transgene into the host chromosome versus the use of episomal expression vectors can greatly affect secretion and, with reference to Parekh & Wittrup, 1997, Biotechnol. Prog., 13, 117-122, that stable integration of the scFv gene into the host chromosome using a 6 integration vector was superior to the use of a 2 μm-based expression plasmid. Parekh & Wittrup, op. cit., had previously taught that the expression of bovine pancreatic trypsin inhibitor (BPTI) was increased by an order of magnitude using a 6 integration vector rather than a 2 μm-based expression plasmid. The 2 μm-based expression plasmid was said to be counter-productive for the production of heterologous secreted protein.
Bao & Fukuhara, 2001, op. cit., reported that “It was first thought that the KlPDI1 gene might be directly introduced into the multi-copy vector that carried the rHSA expression cassette. However, such constructs were found to severely affect yeast growth and plasmid stability. This confirmed our previous finding that the KlPDI1 gene on a multi-copy vector was detrimental to growth of K. lactis cells (Bao et al, 2000)”. Bao et al, 2000, Yeast, 16, 329-341, as referred to in the above-quoted passage of Bao & Fukuhara, reported that the KlPDI1 gene had been introduced into K. lactis on a multi-copy plasmid, pKan707, and that the presence of the plasmid caused the strain to grow poorly. Bao et al concluded that over-expression of the KlPDI1 gene was toxic to K. lactis cells. In the light of the earlier findings in Bao et al, Bao & Fukuhara chose to introduce a single duplication of KlPDI1 on the host chromosome.
Against this background, we had previously surprisingly demonstrated that, contrary to the suggestions in the prior art, when the genes for a chaperone protein and a heterologous protein are co-expressed on a 2 μm-family multi-copy plasmid in yeast, the production of the heterologous protein is substantially increased.
Our earlier application, which has been published as WO 2005/061718, from which this application claims priority, disclosed a method for producing heterologous protein comprising:                (a) providing a host cell comprising a 2 μm-family plasmid, the plasmid comprising a gene encoding a protein comprising the sequence of a chaperone protein and a gene encoding a heterologous protein;        (b) culturing the host cell in a culture medium under conditions that allow the expression of the gene encoding the chaperone protein and the gene encoding a heterologous protein;        (c) purifying the thus expressed heterologous protein from the culture medium; and        (d) optionally, lyophilising the thus purified protein.        
As discussed above, Bao et al, 2000, Yeast, 16, 329-341 reported that over-expression of the K. lactis PDI gene KlPDI1 was toxic to K. lactis cells. Against this background we have surprisingly found that, not only is it possible to over-express PDI and other chaperones without the detrimental effects reported in Bao et al, but that two different chaperones can be recombinantly over-expressed in the same cell and, rather than being toxic, can increase the expression of proteins, including heterologous proteins, to levels higher than the levels obtained by individual expression of either of the chaperones. This was not expected. On the contrary, in light of the teaching of Bao et al, one would think that over-expression of two chaperones would be even more toxic than the over-expression of one. Moreover, in light of some initial findings which are also reported below in the present application, it was expected that the increases in heterologous protein expression obtained by co-expression with a single chaperone would be at the maximum level possible for the cell system used. Therefore, it was particularly surprising to find that yet further increases in protein expression could be obtained by co-expression of two different chaperones with the protein.